A modern internal combustion engine comprises a number of servo controlled actuators, which are controlled by an electronic control unit to automatically modify the position of corresponding engine adjustment or control members.
When an adjustment or control member is to be moved only between two limit positions, i.e. when the adjustment or control member is to take only two positions, non-progressive actuators are used, which may be either electric actuators or non-balanced pneumatic/hydraulic actuators (the choice depends on requirements of cost, size, power and speed).
When the adjustment or control member is to be able to take all the positions comprised between two limit positions, progressive actuators, such as electric or balanced pneumatic/hydraulic actuators, are used (the choice depends on requirements of cost, size, power and speed). In the case of small or medium powers, both the electric actuators and the balanced pneumatic/hydraulic actuators are particularly expensive and cumbersome; specifically, the electric actuators are cumbersome and expensive also due to the presence of control electronics, while the pneumatic/hydraulic actuators are cumbersome and expensive due to the presence of solenoid valves and pneumatic/hydraulic circuits thereof.
Patent application EP1770264A1 describes a progressive displacement control actuator comprising an element which moves along a linear path, a pneumatic or hydraulic driver for displacing the mobile element along the linear path, an adjustment chamber, which accommodates part of the mobile element and is filled with a rheological liquid, and a driving device, which is adapted to apply a variable voltage to the rheological liquid contained in the adjustment chamber to vary the viscosity of the rheological liquid between a minimum value, at which the displacement of the mobile element is essentially free, and a maximum value, at which the displacement of the mobile element is blocked.
U.S. Pat. No. 6,234,060B1 describes a progressive displacement control actuator comprising an element which moves along a circular path, a pneumatic driver for displacing the mobile element along the circular path, an adjustment chamber, which accommodates part of the mobile element and is filled with a rheological liquid, and a driving device, which applies a variable voltage to the rheological liquid contained in the adjustment chamber to vary the viscosity of the rheological liquid between a minimum value, at which the displacement of the mobile element is essentially free, and a maximum value, at which the displacement of the mobile element is blocked.
The progressive displacement control actuators described in patent application EP1770264A1 and in U.S. Pat. No. 6,234,060B1 are relatively complex and expensive, do not allow to obtain a particularly accurate control of the position of the mobile element along the corresponding path and present relatively low energy efficiency and response promptness.